CARBOCATIONIC COPOLYMERIZATION IN THE PRESENCE OF ELECTRON PAIR DONORS .1. COPOLYMERIZATION OF ISOBUTYLENE AND ISOPRENE WITH THE CUMYL ACETATE BCL3 INITIATING SYSTEM

The copolymerization of the isobutylene (IB)-isoprene (IP) monomer pair by the cumyl acetate (CumOAc)/BCl3 system has been reinvestigated, and the earlier proposition1 in regard to the living nature of this system is questioned. In line with recent results on living carbocationic homopolymerizations initiated by tert-ester and -ether/MtX(n) complexes, it is proposed that this copolymerization is initiated by the Cum+ cation arising either directly by the ionization CumOAc.BCl3 --> Cum+-OAcBCl3 or in two steps by the sequence CumOAc + BCl3 --> CumCl + BCl2OAc + excess BCl3 half-arrow-right-over-half-arrow-left Cum + -BCl4 + BCl2OAc. Propagation is ionic in character and termination leads to a chlorine-capped copolymer by collapse with the counteranion. Depending on the penultimate unit, reionization may occur and the chlorine-capped dormant polymers are in dynamic equilibrium with active propagating species: poly(IB-IP)-Cl + BCl3 half-arrow-left-over-half-arrow-left poly(IB-IP)+ -BCl4. The carbocation-stabilizing role of BCl2OAc, an in situ electron pair donor, in isobutylene homopolymerizations was discussed in detail.2 While BCl2OAc mediates living IB homopolymerization under certain conditions, it is of insufficient nucleophilicity to yield living IB-IP copolymerization. IP invariably accumulates during copolymerization so that the earlier published reactivity ratio r(IP) = 4.5(1) is in need of revision. Mechanistic considerations suggest that this complex copolymerization system cannot be described by a single pair of reactivity ratios, r(IB) and r(IP).